1. Field of the Invention:
The present invention relates to the steam cracking of hydrocarbons, optionally in the presence of hydrogen.
2. Description of the Prior Art:
It is known to this art that the steam cracking of hydrocarbons, or cracking in the presence of steam, was used in the United States between 1930 and 1940 for the production of ethylene and aromatic hydrocarbons.
That process was one in which the charge or feedstock to be cracked, in a dilute state, was circulated within tubes which were externally heated. Technological advances made it possible to increase production capacities and to use increasingly heavy hydrocarbon feedstocks.
It too is known that, in such a process, the steam performs a number of functions. Thus, according to B. Blouri (Etat des connaissances sur le traitement thermique des produits lourds ["State of knowledge regarding the thermal treatment of heavy products"]- Revue de l'Institut Francais du Petrole, volume 36, No. 1, January-February 1981), the introduction of steam has three main functions to perform:
(1) It reduces the partial pressure of the hydrocarbon and thus promotes selective cracking in respect of olefins;
(2) it reduces the partial pressure of the high molecular weight aromatic hydrocarbons and reduces their tendency to form coke with the cracking charge and to deposit heavy residues on the face surfaces of the exchangers and the conduits; and
(3) It has a sufficient oxidizing effect on the metal pipes to give a significant reduction in the catalytic effects of the iron or nickel, for the formation of carbon.
A different process is also known to this art, the goal of which is to replace the steam with hydrogen under pressure. The focus of such process is to produce a large proportion of alkenes by cracking a charge without use of a catalyst, in a very short residence time and at a temperature which, if the charge is naphtha, is on the order of 930.degree. C.
The use of hydrogen provides two advantages: the conversion of heavy products into light products is improved, and there is a reduced formation of deposits.
Unfortunately, the presence of a large amount of hydrogen gives rise to hydrogenation reactions which take place in competition with the cracking reactions in a strict sense. The formation of alkenes then suffers from such an inhibitory effect.
Finally, irrespective of the situation, the parasitic effects of exposed wall members, e.g. encrustation, different reaction kinetics with production of varying reaction products, etc., are a constraint in steam cracking processes.
Generally, an effort is made to avoid using multi-tubular reactors. It has long been proposed that cracking should be carried out by direct contact with combustion gases, for example, by means of thermal shock as in U.S. Pat. No. 2,790,838. More elaborate means of contact have also been proposed. Thus, U.S. Pat. No. 4,136,015 provides for introducing the feedstock in atomized form into the gaseous stream produced by combustion of the heavy fuel in the presence of an oxidant, in a burner and mixing zone, followed by a cracking reaction zone and a quenching zone.
The reactor is in the configuration of a convergent-divergent nozzle.
However, use of such a reactor assumes that a complex pattern of flow of fluids can be controlled, as pointed out by Barendregt, Information Chimie, No. 231, pages 217-221 (November, 1982), i.e.:
"The fundamental condition for any cracking process with the production of olefins is suitable control in respect of the residence time/temperature ratio, that control effect being possible in a tubular reactor. As recent developments are in the direction of very short residence times, it will be more difficult to create that fundamental condition and the development of new technologies will be found to be an essential factor."
However, the immediately aforesaid implies that the mixture between a viscous liquid, at the beginning, and a gaseous phase, is homogenously produced over a period of time which is substantially less than the residence time, namely, that the spraying, vaporization and mixing time is of minimum duration.
Also to be added to such disadvantages and drawbacks are those raised by the necessity for changes in feedstock, insofar as the process involves using charges ranging from the lightest to the heaviest and insofar as the process is to be controlled in dependence on the final product which is to be the principal product of the process, e.g., whether ethylene or some other product.
Thus, in spite of the repeated efforts and the many solutions proposed over a period of some fifty years, the basic problems involved in steam cracking have still not been satisfactorily solved.